ci: use actions/cache for ccache on Windows runners

WarpBuilds/cache does not support Windows; keep it for Linux/macOS and
restore the same keys from GitHub's cache service on Windows instead.

.github/workflows/ci.yml

@@ -108,6 +108,7 @@ jobs:
       # flags below pick it up on all three platforms
       # ------------------------------------------------------------------
       - name: Restore ccache
+        if: runner.os != 'Windows'
         uses: WarpBuilds/cache@v1
         with:
           path: ${{ github.workspace }}/.ccache
@@ -116,6 +117,18 @@ jobs:
             ccache-${{ runner.os }}-${{ github.ref_name }}-
             ccache-${{ runner.os }}-
 
+      # WarpBuilds/cache does not support Windows runners; use GitHub's
+      # cache service there (separate backend, same keys)
+      - name: Restore ccache (Windows)
+        if: runner.os == 'Windows'
+        uses: actions/cache@v4
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-${{ runner.os }}-${{ github.ref_name }}-${{ github.sha }}
+          restore-keys: |
+            ccache-${{ runner.os }}-${{ github.ref_name }}-
+            ccache-${{ runner.os }}-
+
       # ------------------------------------------------------------------
       # Configure
       # ------------------------------------------------------------------